Mechanism Associated with Kaolinite Intercalation with Urea: Combination of Infrared Spectroscopy and Molecular Dynamics Simulation Studies

Intercalation of urea in kaolinite was investigated using infrared spectroscopy and molecular dynamics simulation. Infrared spectroscopic results indicated the formation of hydrogen bonds between urea and siloxane/alumina surfaces of kaolinite. The carbonyl group (-C=O) of urea acted as H-acceptors for the hydroxyl groups on alumina surfaces. The amine group (-NH2) of urea functioned as H-donors interacting with basal oxygens on siloxane surfaces and/or the oxygens of hydroxyl groups on alumina surfaces. The H-bonds of urea formed with kaolinite surfaces calculated directly from molecular dynamics simulation was consistent with the infrared spectroscopic results. Additionally, MD simulations further provided insight into the interaction energies of urea with the kaolinite interlayer environment. The calculated interaction energies of urea molecules with kaolinite alumina and siloxane surfaces suggest that the intercalation of urea within kaolinite interlayers is energetically favorable. The interaction energy of urea with alumina surfaces was greater than that with siloxane surfaces, indicating that the alumina surface plays a primary role in the intercalation of kaolinite by urea. The siloxane surfaces function as H-acceptors to facilitate the intercalation of urea. The present study offers a direct view of the specific driving force involved in urea intercalation in kaolinite. The results obtained can help develop appropriate protocol to intercalate and delaminate clay layers for clay-based applications and products.